The present invention relates to a semiconductor device whose connection holes such as via holes or contact holes have characteristic features and a method for producing the semiconductor device. More particularly, the present invention relates to a semiconductor device whose connection holes such as via holes or contact holes produced through a so-called tungsten blanket CVD method have characteristic features and a method for producing the same semiconductor device.
Recently, integration and performance of semiconductor devices have been higher and higher in VLSI circuits and ULSI circuits, and hence a ration occupied by a wiring portion in each semiconductor device tends to be increased. Accordingly, multi-layer wiring becomes a necessary technique in order to prevent an increase in the semiconductor element area. In the semiconductor devices, via holes in which wiring material is embedded are formed to connect the multi-wiring layers with each other. Otherwise, contact holes in which wiring material is embedded are formed to connect an impurity material diffusion region formed on a semiconductor substrate and an upper conductive region with each other.
In semiconductor integrated circuits such as next generation or even later ultra-super LSI circuits, fineness and integration are remarkably advanced. An opening diameter of via holes or contact holes (hereinafter referred to as connection holes) is smaller and smaller down to, for example, 0.35 .mu.m. As the opening diameter becomes smaller, it becomes impossible to form the connection holes with a high reliability by a conventional sputtering method which uses aluminum or aluminum alloy (hereinafter r referred to as an Al system alloy) in view of step coverage problems.
A so-called blanket CVD method is noticed in which a dielectric layer is formed on an impurity material diffusion region, respective electrodes or a lower wiring layer formed on a semiconductor substrate (these layers are sometimes called collectively a lower conductive layer), and conductive material is embedded in opening portions formed in these dielectric layers to form fine connection holes. More specifically, the blanket CVD method provides that, for example, after a tungsten layer made of tungsten has been precipitated by a CVD method, the tungsten layer formed on the dielectric layer is removed through etching so that metal plugs are formed in the opening portions to complete the connection holes. This methods will be hereinafter referred to as a tungsten blanket method.
In the case where the tungsten layer is formed by the tungsten blanket CVD method, it is necessary to form a contact layer under the tungsten layer. The reason for this is that the tungsten layer formed by the tungsten blanket CVD method is superior in step coverage but inferior in contactability to the dielectric layer. Also, it is necessary to prevent metallic fluoride gas such as WF.sub.6 that is used as a raw material gas to form the tungsten layer from eroding the lower conductive layer. Furthermore, since the formation of the tungsten layer by the blanket CVD method is carried out at a relatively high temperature, it is necessary to enhance the barrier property to the lower conductive layer.
For those reasons, it is necessary to form between the tungsten layer and the dielectric layer a contact layer made of Ti layer/TiN layer or Ti layer/TiON layer and the like. In this case, the TiN layer or TiON layer is formed on the Ti layer.
Presently, the contact layer has been produced through a sputtering method. When the contact layer is formed by the sputtering method, it is necessary to fix the semiconductor substrate onto a substrate support table by using a retainer jig. The retainer jig is in general used to retain the semiconductor substrate under the condition that edge portions of the semiconductor substrate are covered by the jig. Accordingly, the contact layer is not formed on portions of the semiconductor substrate which are retained by the jig. FIG. 1A is a schematic plan view showing a semiconductor substrate on which the contact layer is formed by the sputtering method. In FIG. 1A, there are shown the semiconductor substrate 100 and the contact layer 102. A hatching is applied to the region where no contact layer is formed.
In the case where the tungsten layer is formed by the tungsten blanket CVD method, if the tungsten layer is formed on the region of the dielectric layer where te contact layer is not formed, the tungsten layer is liable to be separated from the contact-free region. For this reason, when the tungsten layer is formed, it is not applied to the region of the dielectric layer where the contact layer has not been formed due to the covering jig. The covering jig covers not only the contact-free region of the dielectric layer but also a part of the dielectric layer where the contact layer is formed. Thios condition is shown in a schematic plan view of FIG. 1B. The covering jig is shown by reference numeral 104 in FIG. 1B.
When the tungsten layer is formed by the tungsten blanket CVD method by using such a covering jig, there is a problem that the contact layer on the portion which is covered by the covering jig would by separated from the dielectric layer. This problem would be caused by the following phenomena.
Namely, in the tungsten blanket CVD method, first of all, WF.sub.6 and SiH.sub.4 are used as a raw material gas at a low pressure and a tungsten nuclear material layer is formed on a dielectric layer and in an opening portion by utilizing the reduction reaction of SiH.sub.4. Incidentally, this process (hereinafter referred to as a nucleation step) is carried out in order to improve a film thickness uniformness of the tungsten layer on the contact layer. In the nucleation step, the tungsten nuclear material is sufficiently formed on a part of the contact layer that is not covered by the covering jig. However, the low pressure raw material gas is kept at a low pressure, the raw material gas is hardly introduced on to the part of the contact layer that is covered by the covering jig. As a result, little or no tungsten nuclear material is formed on the part of the contact layer that is covered by the covering jig so that the part of the contact layer is exposed.
Subsequently to the nucleation step, the tungsten layer is formed on the tungsten nuclear material and in the opening portion by using high pressure WF.sub.6 and H.sub.2 as the raw material gas by utilizing the reduction reaction of H.sub.2. This process will be referred to as a high speed growth step. In the region where the tungsten nuclear material is sufficiently formed and is not covered by the covering jig, WF.sub.6 is immediately reduced by H.sub.2 to form the tungsten layer. On the other hand, since the pressure of the raw material gas is high, the raw material gas is introduced also onto the part of the contact layer that is covered by the covering jig.
However, since the tungsten nuclear material is not formed on the part of the contact layer that is covered by the covering jig, WF.sub.6 or F generated by the decomposition of WF.sub.6 is reacted with i contained in the i layer that forms the contact layer, before the adsorption separation between WF.sub.6 and H.sub.2 adsorbed on the contact layer. The WF.sub.6 or F is reacted with Ti contained in the Ti layer that forms the contact layer, thereby forming TiF.sub.x. As a result of the formation of TiF.sub.x, the layer separation occurs between the Ti layer and TiN layer or TiON layer. As a result, there is a problem that the contact layer is separated from the dielectric layer. FIG. 2 is a schematically enlarged cross-sectional view showing such a mechanism. Incidentally, in FIG. 2, a gap between the covering jig 104 and the contact layer surface is depicted in an exaggerated manner.
Presently, in order to cope with such a separation of the contact layer, a RTA process (Rapid Thermal Annealing) is carried out at a temperature of about 900.degree. C. for thirty minutes in a nitrogen gas atmosphere before the tungsten blanket CVD method is carried out and after the contact layer is formed through the sputtering method. through the RTA process, the Ti contained in the i layer is reacted to be TiN so that the TiN layer or TiON layer may suppress the diffusion of WF.sub.6 or F.
However, there is a problem that a crack would be generated in the contact layer depending upon the kind of the dielectric layer used upon the RTA process. Also, in the case where the lower conductive layer is made of Al system alloy, since the Al system alloy is molten, it would be impossible to carry out the RTA process itself. Furthermore, when the RTA process is carried out, the contact resistance between the connection holes and the lower conductive layer would be increased, or the contact between the connection holes and the lower conductive layer would exhibit the non-ohmic characteristics.